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Science: Atomic lens

22 February 1992

By
JEFF HECHT in
BOSTON

Focusing light is easy, but a group of German physicists has performed
the more difficult trick of focusing a beam of helium atoms.

Oliver Carnal and his colleagues at the University of Konstanz used
a device called a Fresnel zone plate, which exploits the wave-like nature
of the atoms (Physical Review Letters, vol 67, p 3231). A Fresnel zone plate
works in the same way as a Fresnel lens, which can be used to focus light.
It consists of an alternating sequence of transmitting and absorbing rings,
which grow thinner as their radius increases.

Using a zone plate which was only 210 micrometres across, Carnal and
his colleagues formed an image of a double slit in a screen. A similar feat
has been achieved independently by David Pritchard and Chris Ekstrom of
the Massachusetts Institute of Technology.

Atomic focusing is more than a demonstration of elegant physics. It
could eventually be used to make a probe capable of imaging surfaces in
great detail. Carnal’s group believes that atomic microprobes could discern
features as small as a nanometre across, using beams with energies of only
0.01 electronvolts.

Electron microscopes can also record very detailed images, but to produce
the correspondingly small wavelengths that are needed for this, it is necessary
to use very high-energy electrons. These can damage the surface under observation.
Because atoms are more massive, short wavelengths can be achieved with less
energy, reducing the chance of damage.